Please explain my scale frustration!

S

Sevens

New member
This is not the end of the world... I'm not even requesting a "fix", but I would sure like to hear a lucid explanation that makes sense.

I'm not scholar and I didn't do well in physics, but I like to think of myself as having the ability to understand something if it's explained to me.

Without further ado, my scale is peeving me off just a bit.

It's a Dillon Eliminator beam scale, likely made by Ohaus, it's probably the exact same thing as a RCBS 505 or any number of other branded scales, but with Dillon blue paint on it. It's your classic beam scale made for handloading.

Here's a pic found on the web -- mine is exactly the same thing:

{Edit: Please read the board policy on posting copyrighted materials. That photo had the copyright notice right on it. You must get permission from the copyright holder to post it.}

When I zero the scale, I'm good to go, but if I place the pan on the hanger slightly different, it'll read plus or minus 2 or 3 tenths of a grain.

I can measure the same exact powder charge, never dumping it from the pan, but if I place the pan on the hanger at a slightly different angle, it'll read low or high by 1, 2, or maybe 3 tenths of a grain.

The pan is NOT binding on anything, nor is the beam. It's on a solid foundation. If you place the pan in the exact same spot, the scale is spot-on all day, every day and never gives a hicccup.

The pan is not touching anything else. And I'm not talking about twisting the pan 180 degrees or even 90 degrees. I'm talking about turning it a touch.

I guess I need a physics teacher or a scale expert to explain why if I rotate the pan an eighth of an inch one way or the other, the scale won't read the same weight.
 
It's pretty simple, actually. Weight X distance, i.e. weight of the beam and counterbalance weight X COM (center of mass) distance from the fulcrum = the weight of the pan X its COM distance from the fulcrum. When you put the pan on the hanger off-center, it changes the COM distance from the fulcrum. Yes, very slightly, but enough to show on the scale.
 
It's pretty simple, actually. Weight X distance, i.e. weight of the beam and counterbalance weight X COM (center of mass) distance from the fulcrum = the weight of the pan X its COM distance from the fulcrum. When you put the pan on the hanger off-center, it changes the COM distance from the fulcrum. Yes, very slightly, but enough to show on the scale.
Click to expand...

I agree that's the only plausible explanation, but I don't think it's that simple. The whole point of having a hanging pan, as opposed to, say, a little fixed platform that you put the pan on, is to avoid this very problem. The distance from the fulcrum to the COM of the mass being weighed shouldn't be a function of where the pan is placed - it should be fixed at the point that the pan hanger is hooked onto the little wire loop. If the pan is off center - which it will almost always be by some amount - the hanger simply swings slightly until the mass is hanging straight below the point of attachment, and the COM distance remains constant. That's how the design is supposed to work, but apparently the system doesn't work perfectly.

I've never tried moving the pan around on my vintage 505, but I'll have to see if it behaves the same way.
 
I use the Dillon scale and have no problem. The pan goes in the same position, every time. Consistancy. I even keep a line level on my scale to make sure it's always level.
 
Ditto above. Shifting the pan's COM performs a similar function to moving the pan further down the balance beam.

You can also test another variance- shift a test powder charge in the pan. Put it all closer to the scale's balance point, and then try it with the powder stacked up as far away from the balance point as possible. You'll probably find a measurable difference.

Now for the physics guys: does a pan with less mass/weight reduce the impact of operator error (inconsistent placement of the pan) on a balance beam scale?
 
OK, I just ran a little experiment with my RCBS 505, which has been in pretty much daily use for the last 35+ years.

First, I noticed that the pan hanger and removable pan are shaped such that the pan nests into the bottom of the hanger exactly the same way every time and you'd really have to go out of your way to not get the pan perfectly centered. But that's not really what I was interested in - I wanted to test whether moving a weight around in the pan would change the reading, or whether, as I had suggested above, the COM doesn't change because it's determined by where the hanger hooks on the the beam at the little wire loop.

I didn't need to use the removable pan for this, so I simply took it off and worked only with the hanger pan. I found a suitable reasonably heavy weight - happened to have a .45 250 gr RNFP cowboy bullet on the bench, so that's what I used. I first centered the weight on the pan and then carefully zeroed the scale. I then removed the weight and placed it back in the center, not trying at all to duplicate exactly where it was before - the scale returned to a perfect zero.

I then moved the weight as far back on the pan as I could, maybe a half inch or a bit more. The pan noticeably moved toward me as the hanger swung to move the COM under its attachment point, and the scale returned to a perfect zero. I repeated that procedure at the other three 90-degree points around the pan. In each case, the pan swung to center the COM - i.e., in the direction opposite where I placed the weight - and the scale returned to a perfect zero - by which I mean the two inscribed lines matched up perfectly, as they had when I originally zeroed the scale with the weight centered.

So, a couple of observations. First, as I mentioned above, the point of the hanging pan is to adjust for any off-centering of the mass being weighed, and it should not matter where the weight is placed in the removable pan, or where the removable pan is placed on the hanging pan. The distance from the fulcrum to the mass being weighed is of course critically important, but that distance is fixed by the point on the beam that the hanging pan is attached at, not where the weight is placed on the pan. Second, I'm beginning to think that the OP has some other problem or a defective scale (balance, really, as the physics guys know).
 
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Sevens, maybe you could try hanging the pan holder from the beam by a very short loop of very fine monofilament fishing line. That should eliminate any slight changes in the location of the point where the mass of the pan/holder is being exerted on the beam. I agree with Scorch that those tiny changes in the distance from the fulcrum are responsible for what you are observing.

DD
 
Mine was the hook was bent & moving on the stirrup hooked to the beam itself . thus changing COM

I fashioned the hook so it had a definite connection point .

It took a while but I wondered why I got em so cheap.

& keep the dust cleaned from the fulcrums !!
 
Flyfish is right...it should not matter what angle the pan is placed as long as the hanger location on the beam isn't affected.

I would look carefully at the thin wire loop that hangs from the beam for an answer. It must not be hanging free.
 
& keep the dust cleaned from the fulcrums !!
Click to expand...

And not the slightest bit of lubricant of ANY kind.

The knife edges need to be clean and dry (and of course sharp).

It does not take very much dust to disturb operation.

We are measuring to very high precision with these scales.

Even a nearby static charged object can attract the pan enough to change the reading.
 
This is a problem that electronic balances can have it the transducer mount is loose or broken, but I haven't seen it on a beam.
Don't like beams, but they "should" be designed to eliminate this problem.
There is something about the OP's pan mount and hook that is off.
 
To The OP.
With rubbing alcohol and Q-tips you can
try cleaning the pivot knives on the beam, Don’t forget the
back ones where the pan yoke is. Then with a twisting motion
Run the q-tips on the agate bearings...

Don't ask me how I know this, But,:rolleyes:
Just on a whim, have you ever “Cleaned up” the scale or wiped
It down with light oiled rag or anything?? If so, wash with hot water
And Dawn followed by an alcohol bath and a blow dryer. :D

HTH
 
That's an excellant scale, your problem makes no sense.

The location of the pan on the hanger is transfered to the beam's loop by the hanger's hook, it should make no difference at all how the pan sits on the hanger.

Makes me wonder if you aren't moving something on the beam's hanger loop that does shift the position of mass relitive to the beam; clean it too.
 
Actually it looks like the top end tab of your hanger is too long and is catching on the 1/10 adjustment tab. I would file off some of that hanger end so that it is free from touching anything!
 
Looking at your picture, all your sliders are set to 0 but your scale isn't reading 0. Do you call it "zeroed" set like that ? If so, that could be your problem.
 
I also have a 505 and experienced a simular problem. There are a couple of issues that cause this. Make sure the contact point on the beam where it sits into the scale body is clean, and not flattened or dinged. Also the spot it sits into must be free of debris and not dinged. Then check the hanger pins and the hanger wire and make sure both are smooth and clean. My problem was the V shapped beam rests, they had a build up of crud and it was causing friction or binding that in turn effected how the beam would rest. I may not have explained things real well, but I think you get the idea. Just make sure every point of contact is clean and without anything that would restrct or effect free movement. It doesn't take much to interfere with the tenths of grain weights. Since that experience, I've been covering my scale when not in use to prevent dust from building up and I give the entire scale a dusting in those areas every now and then.
 
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